1. Field of the Invention
The present invention relates to a developing apparatus for developing a visual electrostatic latent image on an electronic photographing or an electrostatic recording apparatus and, in particular, a developing apparatus capable of obtaining a high-quality image using a one-component developing agent.
2. Description of the Related Art
In recent years, a developing system using a one-component developing agent has been reduced to practice.
This system, as distinct from a system using a two-component developing agent comprised of a toner and carrier, does not require any developing agent carrier, magnet roller, toner density control device, etc. and can be made compact and low in cost.
Conventionally, impression development has been known as one of the developing methods using a one-component developing agent.
This method is disclosed in U.S. Pat. No. 3,152,012, U.S. Pat. No. 3,731,146, Published Unexamined Japanese Patent Application 47-13088 and Published Unexamined Japanese Patent Application 47-13089, in which a toner carrier having a toner (developing agent) layer formed thereon is set in contact with an electrostatic latent image on a drum at a relative peripheral speed of substantially zero.
The method above ensures a compact apparatus of a simpler arrangement and better toner color development, because it is not necessary to employ a magnetic material.
In compression development, since the toner carrier is set in contact, or pressure contact, with the electrostatic latent image on the drum surface, it is necessary to use a developing roller of proper elasticity and conductivity as the toner carrier. In particular where the electrostatic latent image carrier is comprised of a rigid body, it is necessary that the developing roller be made of an elastic body so as to prevent any injury to the rigid body.
U.S. Pat. No. 3,866,574, U.S. Pat. No. 3,893,418 and other prior art disclose a non-contacting type developing method as the one-component type developing method.
In this method, a developing agent layer is set in proximity to an electrostatic latent image on a drum and development is made by the flying of the toner at the latent image with an AC field applied across the developing agent layer and the latent image. In this case, it has been usual to form a developing roller as a rigid body.
In the one-component developing method it is generally known that an image defect emerges due to a hysteresis image called a "sleeve ghost."
The "sleeve ghost" is caused primarily by the toner layer formation quality at the time of forming a toner layer on the developing roller. Unless the thickness of the toner layer, the amount of charges involved, etc., at some area, are the same as those at other areas before one toner-transferred area on the electrostatic latent image side is again used in the next development cycle, there occurs a hysteresis density difference between these two areas.
A new problem arises in the case where the developing apparatus is applied to the so-called developing/cleaning process (cleanerless process). That is, a foreign substance, such as paper dust, separated from a transfer sheet, upon cleaning, together with the collected developing agent is mixedly returned back into the developing apparatus. In this case, the foreign substance accumulates on the developing roller (developing agent conveying means) so that the developing agent is not properly electrically charged. This poses a problem. The apparatus involved must be replaced with a new one or cleaned due to image defects in spite of the presence of the developing agent.
In the prior art, a fairly good solution has been proposed, as will be set out below, in the case where the developing apparatus is used as an independent apparatus.
(1) A scraping member, such as a blade or a sponge roller, is set in contact with the developing roller to scrape all residual toner off the developing roller, so that any hysteresis difference as already set out above is removed.
(2) A bias voltage is applied to, for example, a conductive roller, etc., at which time the conductive roller is moved near the developing roller to separate the toner off the whole surface of the developing roller.
In either case, after the development of the toner, the residual toner is temporarily separated off the whole surface of the roller, so as to erase a hysteresis difference resulting from a development pattern. This approach is reasonable, taking the cause of that phenomenon into consideration.
These approaches still involve some problems to be solved.
Setting a member, such as a blade, in abutting contact with the developing roller is not desirable from the standpoint of its service life, because the injury of the developing roller, sticking of some toner on the roller surface, etc. are involved. These defects are particularly prominent in the case where an elastic roller comprised of rubber or resin is used as the developing roller. From this, it is considered preferable to employ a roller-like scraping member as a separation member for separating the developing agent from the elastic developing roller. In spite of such a countermeasure it has been found that irregularities and fogging sometimes occur on an image on the drum surface after 1000 to 2000 copies have been printed out.
As a result of investigations into such a cause, it has been found that, since the separated developing agent is gradually accumulated at an area near the developing agent separation member and a fresh developing agent and once-charged unspent developing agent (collected developing agent) are again supplied, in a mixed state, to the developing roller, an image emerges at an uneven density upon development. Such an image defect occurs due to a hysteresis difference between the new and unspent developing agents in the developing apparatus as in the case of the sleeve ghost (hysteresis image) emerging on the developing roller.
It has been found that, in the case where the developing apparatus is employed as the aforementioned developing/cleaning apparatus, not only those developing agents different in their charging hysteresis are again supplied, in the mixed state, for development but also a foreign substance containing a paper dust (talc, kaolin, etc.) is deposited on the developing roller and separation roller with the separation and collection of the developing agent. These situations cause a charging defect of the developing agent and, hence, the apparatus has a shortened service life.